It had all begun on the elevated. There was a particular little sea of roofs he had grown into the habit of glancing at just as the packed car carrying him homeward lurched around a turn. A dingy, melancholy little world of tar-paper, tarred gravel, and smoky brick.

There was a washed-out advertisement of some ancient patent medicine on the nearest wall. Superficially it was like ten thousand other drab city roofs.

It seemed unusually bleak and suggestive, almost beautifully ugly, though in no sense picturesque; dreary, but meaningful.

One evening toward winter he noticed what seemed to be a shapeless black sack lying on the third roof from the tracks. He did not think about it. It merely registered as an addition to the well-known scene and his memory stored away the impression for further reference.

Its colour and texture, and the grimy stains around it, suggested that it was filled with coal dust, which was hardly reasonable. Then, too, the following evening it seemed to have been blown against a rusty ventilator by the wind--which could hardly have happened if it were at all heavy. Perhaps it was filled with leaves. Catesby was surprised to find himself anticipating his next daily glance with a minor note of apprehension.

What difference did it make if his imagination had played tricks on him, and he'd fancied that the object was crawling and hitching itself slowly closer across the roofs? That was the way any normal imagination worked